An image forming apparatuses such as a printer, a facsimile machine, or a copying machine is an apparatus for forming an image including a character or a symbol on recording medium such as a paper sheet, a cloth sheet, or an OHP sheet based on image information. In particular, an electrophotographic image forming apparatus may form a high-definition image on a plain paper sheet at a high speed, and hence, has widely been used in offices. For such an electrophotographic image forming apparatus, a thermal fixation method has been used widely in which a toner on a recording medium is heated and melted by means of heating and a melted toner is pressed whereby the toner is fixed onto the recording medium. This thermal fixation method may provide high speed fixation, high image quality fixation, or the like, and hence, has been used preferably.
However, more than half of the electric power consumption of such an electrophotographic image forming apparatus is consumed for heating a toner in the thermal fixation method. Therefore, a fixation device with a low electric power consumption (energy saving) is desired from the viewpoint of environmental conservation in recent years. That is, a fixation method is desired in which a temperature at which a toner is heated in order to fix the toner is extremely lowered more than ever or it is unnecessary to heat a toner. In particular, a non-heating fixation method for fixing a toner on a recording medium without heating the toner at all is ideal from the viewpoint of low electric power consumption.
For such a non-heating fixation method, for example, Japanese Patent No. 3,290,513 proposes a wet toner fixation method wherein an oil-in-water-type fixing agent in which an organic compound being capable of dissolving or swelling a toner and being insoluble or difficult to be dissolved in water is dispersed and mixed in water is sprayed or dropped onto a surface of a substrate to be subjected to fixation on which an unfixed toner is arranged at a predetermined position so that the toner is dissolved or swelled, and subsequently the substrate to be subjected to fixation is dried out.
However, when a large mount of a fixing agent is provided to an unfixed toner in the wet fixation method of Japanese Patent No. 3,290,513, a recording medium (substrate to be subjected to fixation) such as a transfer paper sheet may absorb water contained in the fixing agent so as to cause cockle or curl of the recording medium, because an oil-in-water-type fixing agent is used in which an organic compound being insoluble or difficult to be dissolved in water is dispersed and mixed in water. Thereby, stable and high-speed conveyance of a recording medium as may be required for an image forming apparatus may be impaired significantly. Herein, if a large amount of water contained in a fixing agent is evaporated by using a drying device so as to remove water contained in a fixing agent provided to a recording medium, an amount of electric power may be required which is comparable to an electric power consumption of an image forming apparatus using a thermal fixation method.
Furthermore, some kinds of oily fixation liquids in which a material for dissolving or swelling a toner is dissolved in an oily solvent have conventionally been proposed for a fixation liquid that does not repel an unfixed toner subjected to a water repellent treatment. For one of them, for example, Japanese Patent Application Publication No. 2004-109749 proposes a fixation liquid in which a material for dissolving or swelling a resin component constituting a toner, such as an aliphatic dibasic acid ester as a component, is diluted (or dissolved) in a nonvolatile dimethyl silicone as a diluent (or solvent). Moreover, Japanese Patent Application Publication No. 59-119364 proposes a solution for fixing an unfixed toner image in a miscible state, which is provided by mixing 8 to 120 parts by volume of a silicone oil with 100 parts by volume of a solvent dissolving a toner and having a miscibility with the silicone oil, for a fixation solution that may be used for a fixation method in which an unfixed image formed by means of an electrostatic method may be fixed on an image receiving sheet sharply and readily without disturbing the image. Such an oily fixation liquid contains an oily solvent having a high affinity with an unfixed toner subjected to a water repellent treatment, and accordingly, it may be possible to dissolve or swell a toner and fix the toner on a recording medium without repelling the unfixed toner subjected to a water repellent treatment.
Moreover, Japanese Patent Application Publication No. 2004-109747 and Japanese Patent Application Publication No. 2009-008967 propose techniques for providing a fixation liquid in a foam state so as to reduce the density of the liquid and eliminate the influence of the surface tension of the liquid whereby fixation with an applied fixation liquid whose amount is extremely smaller than a conventional amount may be conducted without disturbing a toner image.
A configuration for providing a fixation liquid to an unfixed toner layer is provided in any of the above-mentioned patent documents, but, in a configuration for applying a fixation liquid to an unfixed toner layer 3 on a recording medium 2 by using an application roller 1 as a contact application part as illustrated in FIGS. 8A and 8B, when the thickness of a fixation liquid layer 4 on the application roller 1 is smaller than that of the unfixed toner layer 3 in order to provide a small amount of the fixation liquid to the recording medium 2 as illustrated in FIG. 8A, an unfixed toner particle(s) may be attracted by a surface tension produced by a liquid film of the fixation liquid on the surface of the application roller 1 at a position at which the application roller 1 leaves the recording medium 2, causing the toner particle(s) to adhere to the surface of the application roller 1, and accordingly, an image on the recording medium 2 may be disturbed considerably.
On the contrary, when the thickness of the fixation liquid layer 4 on the application roller 1 is sufficiently larger than that of the unfixed toner layer 3 as illustrated in FIG. 8B, a surface tension provided by a liquid film on the surface of the application roller 1 may be unable to act on a toner particle(s) directly at a position at which the application roller 1 leaves the recording medium 2 because of a large amount of the liquid and no toner may adhere to the roller, but a large amount of the fixation liquid is applied on a paper surface, and accordingly, a toner particle(s) may be moved by an excess amount of the fixation liquid on the recording medium 2 so as to cause image degradation or to provide a long drying time period thereby causing a problem in its fixation responsiveness. Furthermore, a significant extent of residual liquid feeling (wet feeling when a paper sheet is touched by a hand) is produced on a paper sheet. Moreover, when the fixation liquid contains water and when a large amount thereof is applied to a medium containing a cellulose such as a paper sheet, the medium such as a paper sheet may curl significantly and jamming of a paper sheet may be caused at the time of conveyance of a medium such as a paper sheet in an apparatus such as an image forming apparatus. Hence, it may be very difficult for such a configuration for conducting roller application with a fixation liquid to attain both application of a small amount of a fixation fluid to a toner layer on a paper sheet for improvement of its fixation responsiveness, reduction of a residual liquid feeling or prevention of its curl, and prevention of toner offset onto a fixation roller. Also, when a die coat part, a blade application part, or a wire bar application part is used as a contact application part and when a small amount of a fixation fluid is provided, a toner may adhere to the contact application part due to its surface tension and image degradation may be caused.
As described above, it may be very difficult for a contact application part and a conventional fixation liquid formulation to attain both application of a small amount of a fixation liquid to a toner layer on a paper sheet to improve its fixation responsiveness and its uniform application without disturbing a toner image. Furthermore, the above-mentioned problem is not necessarily limited to a toner on a recording medium but may occur in a configuration for providing a fixation fluid in a liquid state to a resin-containing fine particle layer on a medium in any case.
Meanwhile, an anionic surfactant may be generally suitable for a foaming agent suitable for a foam-like fixation fluid. The foaming property of such an anionic surfactant is maximally exerted for a liquid in a weak-alkaline state in which its pH is 7 or more. Meanwhile, a softening agent for softening a resin such as a toner in a fixation fluid may have an ester group in most cases and hydrolysis of such an ester group may occur in an alkaline state in which its pH is 7 or greater so that the softening agent may be decomposed chemically and the ability of the softening agent may be lost. Hence, if the pH of a fixation fluid is 7 or more to enhance the foaming property of the fluid, a problem may be caused such that a softening agent may be decomposed chemically in a storage container for a long period of time for storage of the fixation fluid causing so that its fixation ability to be lost and the reliability of a fixation device may be impaired. If a weak-acidic state is provided in which its pH is 6 or less, the chemical decomposition of a softening agent may be suppressed, but the property of a foaming agent may be degraded causing foaming of a fixation fluid not be attained well and formation of a foam film to be disturbed, thereby causing defective fixation.